As is known in the art, any change in voltage or current on an electronic signal line causes radiation to be emitted. Slowly shifting currents and voltages cause small amounts of radiation at very long wavelengths, and very rapid changes in voltage or current cause large amounts of short wavelength radiation to be emitted. In general, radio frequency (RF) radiation from integrated circuits (ICs) is a problem because RF radiation is easily detected by outside radio receivers, and is also energetic enough to cause measurable voltage changes on signal lines in other nearby electronic devices.
An electronic device, such as a microprocessor, has numerous signal lines whose electrical states are shifting up and down by as much as 5 volts at MegaHertz to GigaHertz frequencies. Electronic devices include individual ICs, such as memory chips, microprocessors, cross bar switches and logic arrays, multichip modules containing several IC chips, hybrid modules connecting active ICs with passive electrical components such as resistors and capacitors, and electronic systems with circuit boards and back planes connecting the circuit boards and the electronic devices on the circuit boards. The high speed voltage shifting of the signal paths creates RF radiation, at least some of which escapes from the electronic package containing the electronic device and may be detected by nearby radio receivers, or cause unintended voltage changes in nearby electronic devices. Such unintended voltage changes may be of a large enough magnitude to be mistakenly interpreted as a change in a logic state, thus resulting in a logic failure of the nearby electronic device. As a result of these problems, RF emissions from electronic devices must be kept below levels dictated by FCC regulations.
Although RF radiation is emitted by any rapidly changing signal line, the strength of the emitted radiation also depends on the antenna characteristics of the emitting signal line. A typical electrical conductor on an integrated circuit (IC) is not an efficient antenna. A strip of conductive material on the IC is neither a proper dipole antenna nor a quarter wave length monopole at typical RF frequencies. There are many other conductors on the IC having different voltages and phases in extremely close proximity to the emitting signal line. Each of these other conductors is likely to be emitting radiation as well, thus each signal line may represent a radiation shield or a counterbalance to the radiation of the others. In modem microprocessors the synchronized clock lines may produce RF interference at the clock frequency.
However, it is well known that modern high speed ICs have a need to keep the temperature of the semiconductor junctions to a low level for reasons of long term reliability. The semiconductor material of an IC is typically contained within an electronic package that protects the IC and provides electrical connections from the IC to the printed circuit board (PCB) upon which the IC is mounted. The same situation is also true of hybrid packages, and other types of electronic systems.
A typical method used for cooling ICs and other electronic devices is to attach a heat dissipative structure, such as a heat spreader or a heat sink, to the IC's electronic package. Many methods of attaching heat sinks to electronic packages are known in the art, and include glues, spring clips, and other physical attachment means such as bolts and studs. The glues used include various cyano- and methyl-methacrylates, dual and single component epoxy compounds, either liquid or solid, or preformed semisolid epoxy shapes. The epoxy or other glue may be filled with a thermally conductive material, such as silver particles, to improve the thermal conduction, and many single component epoxy products require a heat treatment step to activate the heat sink to package adhesion. Physical heat sink attachment methods may use a compliant thermally conductive medium between the heat sink and the electronic package to fill any air gaps. Heat spreaders may be built into the package or may be attached to the outside of the electronic package by means of soldering, welding or brazing the metallic heat spreader to a metallized portion of the package.
A typical electronic package is composed of a non-conductive material (known as dielectric), such as ceramic or plastic, to allow the electrical signals from the IC to pass along conductors provided within the dielectric of the electronic package, and to the PCB without attenuation and inter-conductor interference. However, most dielectric materials are also poor heat conductors, i.e., they are thermal resistors, and thus a thermal gradient will exist if a heat sink is simply attached to the electronic package. Therefore, it is known to provide another type of heat dissipative structure, called a metallic slug or a thermal slug, that traverses part of the electronic package. The slug provides a better thermal path through the dielectric of the electronic package, and thus improves the thermal conduction path between the backside of the IC and the heat sink. The slug thus reduces the thermal resistance between the heat sink and the IC. Such metallic slugs are typically made of copper for plastic packages, or of a copper/tungsten alloy for ceramic packages.
In either of the above situations, the presence of a heat sink may change the efficiency with which the radiation is emitted. The heat sink will have an RF voltage caused by inductive coupling between the heat sink and the IC or other electronic device. The case discussed above of a heat sink with a metallic slug will provide closer coupling and hence possibly greater transmitted RF radiation levels. The heat sink is a better antenna because of the size of the heat sink relative to the IC signal lines. A heat sink may become an efficient antenna at a system operating clock frequency of about 300 MegaHertz (MHZ) for large sized heat sinks. The heat sink is also not shielded by other signal radiators and it may be a dipole antenna. The metallic heat sink is one dipole arm, with the ground plane of the PCB boards as the second arm. To complete the antenna model there are ground radials formed by the IC power and ground voltage lines. Thus the industry solution to the overheating problem in high speed ICs unintentionally results in an increase in RF noise radiated from the IC.
It is also known in the art to provide grounded conductive shields either totally or partially surrounding, i.e., external to, the heat sink and IC package to reduce the level of emitted RF noise. Shield plates are typically placed as close to the heat sink as possible without making electrical contact to provide the maximum shielding. This is because the shields form what is known as an image charge which contains the RF noise. The closer the shields are located to the radiating source the better the coupling and the closer the image charge will be to exactly canceling out the RF noise.
There exists a problem in the art with external shields. Each shield must be connected as closely as possible with the PCB reference plane (typically the ground plane) so as to provide the lowest RF impedance for optimal image charge formation. Typically, each of the shields will have numerous connections (typically made by solder joints) to the PCB board to minimize unwanted resistance. The requirement of close electrical connection to the PCB results in increased PCB size and complexity, and consequent increases in finished electronic device cost. The physical presence of the shields on the PCB results in an increased distance between components, for example between a microprocessor and its associated cache memory, and the shields may obstruct cooling air flow and result in IC overheating. The increased physical distance between components on the PCB results in a decrease in device speed due to increased time of flight of the electronic signals, increased parasitic capacitance and signal line mutual and self inductance problems.
It is also known in the art to provide an electronic system with a box or enclosure that is grounded to a reference plane to form what is known as a Faraday shield. The shielded box prevents RF radiation from leaving the system and affecting surrounding devices, and also prevents outside RF radiation from affecting the system. However, such Faraday shields are expensive to provide since the size of the allowable cooling air holes in the shield must be small enough to block the RF noise generated by the system, and the trend in the industry toward higher frequency system operation means that the hole size will continue to drop. Such shields also fail to attenuate RF radiation between individual components of the same system.
It is also known in the art to provide the heat sink with electrical connections directly to the PCB reference plane by attaching metallic posts from the four comers of the heat sink to solder pads on the PCB directly below the heat sink. This method requires additional manual assembly operations be added to the IC attachment to the PCB. It also requires additional space on the PCB for the post attachments, and is practical only in cases where heat sink is substantially larger than the IC package.
Therefore it would solve a problem in the art to provide a heat sink RF shielding device that did not require additional connections to the PCB ground plane beyond those provided by the electronic package, did not decrease the density of the PCB, or increase PCB assembly costs by requiring tighter assembly tolerances.